The present invention relates to a semiconductor memory device having a nonvolatile memory.
Heretofore, semiconductor memories storing video data, audio data, or other data (hereinafter referred to as xe2x80x9cvideo and other dataxe2x80x9d) therein have been sold as package media. Primarily, mask ROM media are used as such package media.
Conventional package media have particular codes that are written by the steps of ion implantation and low-layer metal interconnection formation in a process of fabricating semiconductor integrated circuits. Therefore, it generally takes more than about two months to complete the conventional package media.
If an actual number of package media that are sold is much smaller than the number of manufactured package media, then since the mask ROM media are not reprogrammable, there arises a business problem in that the manufacturer has to have a large inventory of unsold package media.
When the demand grows for certain video and other data, it is necessary to manufacture new mask ROM media storing those video and other data, and hence a certain period of time is required before the desired package media can be supplied.
If an electrically reprogrammable nonvolatile semiconductor memory, such as a flash memory, is used for package media, then package media can be manufactured in a short period of time because desired information can be electrically rewritten after the package media have been completed.
Since it is easy to rewrite information stored in those electrically reprogrammable nonvolatile semiconductor memories, desired information can be written therein upon demand, and the manufacturer does not have to keep a large inventory of unsold package media.
However, the nonvolatile semiconductor memories are problematic in that since data stored therein can be rewritten by general users according to a certain procedure, it is not possible to determine whether the manufacturers are responsible or not if the nonvolatile semiconductor memories suffer a defect after data stored in the memories have been modified.
Another problem with the use of the nonvolatile semiconductor memories is that parties other than the manufacturers are given an opportunity to produce the contents, the contents may be exchanged or copied in violation of their copyright, and pirate edition producers tend to be rampant.
It is an object of the present invention to provide a semiconductor memory device which is capable of selectively performing different functions.
According to an aspect of the present invention, there is provided a semiconductor memory device including: a chip having a nonvolatile memory capable of rewriting stored data; and mode switching means disposed on the chip for irreversibly inhibiting data from being written in the nonvolatile memory upon the elapse of a preset period of time from the time when the power supply of the semiconductor memory device is turned on if a predetermined signal is not supplied from outside of the chip within the preset period of time.
According to another aspect of the present invention, there is provided a semiconductor memory device including: a chip having a nonvolatile memory capable of rewriting stored data; and mode switching means disposed on the chip for irreversibly inhibiting data from being written in the nonvolatile memory if the number of times that a power supply of the semiconductor memory device is turned on reaches a predetermined number.
The semiconductor memory device according to the present invention is capable of selectively performing different functions because it provides higher security for data stored in the nonvolatile memory and allows stored data to be selectively rewritten with predetermined signals supplied from outside of the chip within a preset period of time.